There are a number of basic criteria involved in the design and scaling of a hall thruster. The fundamental relationship of the accelerator characteristic length (L) to the ion Larmor Radius (.rho..sub.i), the electron Larmor radius (.rho..sub.e) and ion-neutral mean free path (.lambda.) is defined as ##EQU1## where m is mass, n is number density, q is elementary charge, v is velocity, B is magnetic field and Q is collision cross-section. Subscripts e, i, n denote electron, ion and neutral respectively.
To maintain constant (.rho..sub.i). (.rho..sub.e) (.lambda.) and v.sub.i results in the following scaling relationship. ##EQU2##
These equations relate the thruster characteristic dimension L to plasma parameters and provide the necessary design relationships between the plasma parameters and the thruster geometry. The magnetic circuit is then determined using Gauss and Ampere's law with the final design strongly influenced by structural, thermal and fabrication considerations.
For small thrusters the challenge is to design a magnetic circuit that can handle the required flux while minimizing the increasing dissipation in the electromagnetic coils. As evident from the above scaling relations, the smaller the thruster the stronger the magnetic field, which then requires proportionately more turns on the electromagnet coil leading to higher coil losses through increased Joule dissipation. These small thruster challenges, combined with increasing discharge wall loses (ion and electron collisions with the walls) and increased heat loading stemming from the scaling laws (higher particle, current and power densities), have hampered the development of efficient Hall thruster that operate at nominal power that is less than a few hundred watts. A typical small Hall thruster such as the Russian SPT 50 has an efficiency in the low thirties, Isp in the 1200-1400 sec. range and lifetime of the order of 1000 hours. This performance life is substantially below larger multi KW thrusters.
Conventional Hall thrusters of the SPT and TAL type, regardless of their size, have a magnetic circuit with an inner central stem that guides the magnetic flux into (or out of) the inner pole. To guide the flux without substantial fringing across the plasma gap toward the outer pole (or in the reverse direction) the inner pole diameter is generally larger than the inner stem diameter. This then allows the inner stem, in conventional SPT and TAL type thrusters, to be the core of an inner electromagnetic coil whose outer diameter is smaller than the diameter of the inner pole. As the accelerator size discharge cavity decreases, the inner pole diameter decreases with it, approaching the inner stem diameter, which cannot be reduced as rapidly to accommodate the increasing magnetic field. In the limit, the inner pole and the inner stem have the same diameter with no space for the inner electromagnetic coil. This forces a reconfiguration of the discharge chamber relative to the magnetic circuit.
A common problem with current Hall field plasma accelerators is the fluctuation of the discharge current which may heat up the magnetic circuit by inducing eddy currents within the solid (unsegmented) magnetic material if the discharge is electrically in series with the electromagnetic coil(s). This becomes important in a small thruster with a large magnetic field that may as a consequence have large coil inductance. Also, the discharge current and voltage fluctuations generally require damping using a filter in the power source which increases its complexity, part count, mass and volume.